Introduction
The law of the sea governs the rights and duties of states in relation to the world's oceans. A fundamental aspect of this legal regime is the process of delimitation, which involves defining the extent of a coastal state's jurisdiction over the seas adjacent to its coast and resolving overlapping maritime claims with neighbouring states. The framework for this process is primarily established by the United Nations Convention on the Law of the Sea 1982 (UNCLOS), which is a comprehensive treaty that is widely considered to reflect customary international law. This essay will explain the legal principles and methods used for the delimitation of maritime zones and the settlement of maritime boundaries. It will first outline how the different maritime zones are measured from the coastal state's baseline, before examining the legal principles applied to delimit overlapping zones between adjacent or opposite states. The central argument is that UNCLOS provides a structured-yet-flexible system, combining fixed rules for defining zones with equitable principles for resolving boundary disputes.
The Role of Baselines
The starting point for measuring the breadth of all maritime zones is the baseline. As Tanaka (2019) explains, the baseline acts as the dividing line between a state's internal waters (such as rivers and bays) and its outer maritime zones. UNCLOS provides for two main types of baselines.
The primary method is the 'normal baseline', defined in Article 5 of UNCLOS as "the low-water line along the coast as marked on large-scale charts officially recognized by the coastal State". This method is straightforward and involves following the natural contours of the coast at low tide. This is the default rule and is used for coasts that are relatively simple and regular in shape.
However, for coasts that are deeply indented, cut into, or fringed with islands, the normal baseline method is impractical. For these situations, Article 7 of UNCLOS permits the use of 'straight baselines'. This method allows a state to draw straight lines joining appropriate points along the coast, creating a simpler and more stable baseline from which to measure maritime zones. The waters on the landward side of the straight baseline are considered internal waters. The legitimacy of this method was established in customary international law by the International Court of Justice (ICJ) in the Anglo-Norwegian Fisheries Case (United Kingdom v Norway) [1951] ICJ Rep 116, which found that Norway's long-established system of straight baselines was not contrary to international law. UNCLOS codified this practice but added conditions in Article 7(3): the straight baselines must not depart to any appreciable extent from the general direction of the coast, and the sea areas lying within the lines must be sufficiently closely linked to the land domain to be subject to the regime of internal waters.
The Principal Maritime Zones
Once established, the baseline is used to determine the outer limits of a state's maritime zones, each of which carries a different legal status and set of rights for the coastal state.
Territorial Sea
The territorial sea is the belt of sea immediately adjacent to the coast, over which the coastal state exercises full sovereignty, similar to its land territory. This sovereignty extends to the airspace above and the seabed and subsoil below (UNCLOS, Article 2). The one major limitation on this sovereignty is the right of innocent passage for foreign ships, as defined in Articles 17-19 of UNCLOS. The maximum breadth of the territorial sea is set at 12 nautical miles (nm) from the baseline (UNCLOS, Article 3). Most coastal states have claimed a 12nm territorial sea.
Contiguous Zone
Beyond the territorial sea, a coastal state may claim a contiguous zone. This is not a zone of sovereignty, but rather one of limited jurisdiction. Within this zone, the coastal state may exercise the control necessary to prevent or punish infringement of its customs, fiscal, immigration, or sanitary laws and regulations within its territory or territorial sea (UNCLOS, Article 33). The contiguous zone may not extend beyond 24nm from the baselines from which the breadth of the territorial sea is measured. It is an optional zone, and a state must specifically claim it.
Exclusive Economic Zone (EEZ)
The Exclusive Economic Zone (EEZ) is a zone extending up to 200nm from the baselines. Within the EEZ, the coastal state does not have sovereignty, but has sovereign rights for the purpose of exploring, exploiting, conserving, and managing the natural resources, whether living or non-living, of the waters, seabed and subsoil (UNCLOS, Article 56). This includes rights over fishing, mining, and the production of energy from wind, water, and currents. The EEZ concept was one of the major innovations of UNCLOS, balancing the resource interests of coastal states with the freedoms of navigation and overflight for all states (Churchill and Lowe, 2015).
The Continental Shelf
The continental shelf comprises the seabed and subsoil of the submarine areas that extend beyond a state's territorial sea throughout the natural prolongation of its land territory. The coastal state has sovereign rights over the continental shelf for the purpose of exploring and exploiting its natural resources, such as oil and gas deposits (UNCLOS, Article 77). Every coastal state has a continental shelf extending at least 200nm from its baselines, regardless of the geological characteristics (UNCLOS, Article 76(1)). In certain circumstances where the physical shelf extends further, a state may be able to claim a continental shelf beyond 200nm, subject to a complex set of scientific and legal criteria and review by the Commission on the Limits of the Continental Shelf (CLCS).
Delimitation of Overlapping Boundaries
While the outer limits of zones are clearly defined, disputes often arise when the potential maritime zones of neighbouring states (either adjacent or opposite each other) overlap. UNCLOS provides different rules for different zones.
For the territorial sea, Article 15 of UNCLOS states that where the coasts of two states are opposite or adjacent, neither is entitled to extend its territorial sea beyond the median line (a line equidistant from the nearest points on the baselines of the two states). This is a default rule that can be overridden by agreement or the existence of a historic title or other special circumstances.
For the delimitation of the EEZ and the continental shelf, the rules are more flexible. Articles 74 (EEZ) and 83 (Continental Shelf) of UNCLOS contain identical provisions stating that delimitation shall be effected by agreement on the basis of international law "in order to achieve an equitable solution". This wording represents a compromise between states that favoured a strict equidistance rule and those who argued for a more flexible approach based on 'equitable principles'. The North Sea Continental Shelf Cases (Federal Republic of Germany v Denmark; Federal Republic of Germany v Netherlands) [1969] ICJ Rep 3 was a key case in this area, where the ICJ rejected the argument that the equidistance principle was a mandatory rule of customary law, instead emphasising that delimitation must be based on equitable principles.
Over time, international courts and tribunals have developed a standard methodology for achieving this "equitable solution". This is often referred to as the three-stage approach (Guilfoyle, 2021).
- Provisional Equidistance Line: The first step is to draw a provisional line where every point is equidistant from the nearest points on the baselines of the two states concerned. This serves as a starting point.
- Adjustment for Relevant Circumstances: The second step involves considering whether there are any 'relevant circumstances' that would make the provisional equidistance line inequitable. If such circumstances exist, the line is adjusted. A wide range of factors can be considered relevant, including the general configuration of the coasts, the significant disparity in the lengths of the relevant coastlines, the presence and location of islands, and historical resource claims. The aim is to ensure the boundary reflects the geography of the situation fairly. Courts have shown a reluctance to treat economic factors as relevant circumstances, focusing primarily on geographical considerations.
- Disproportionality Test: The final stage is a check to ensure that the adjusted line does not lead to a result that is significantly disproportionate. The court will compare the ratio of the relevant maritime areas awarded to each state with the ratio of their relevant coastal lengths. This is not a tool for precise mathematical apportionment but an overall check for gross disproportionality to ensure the final solution is genuinely equitable. This three-stage method was clearly applied in cases like Maritime Delimitation in the Bay of Bengal (Bangladesh/Myanmar), ITLOS Case No. 16, Judgment of 14 March 2012.
Conclusion
The delimitation of maritime zones and boundaries is a complex process governed by a detailed legal framework primarily contained within UNCLOS. The system begins with the establishment of baselines, which act as the foundation for measuring the breadth of the territorial sea, contiguous zone, EEZ, and continental shelf. This provides clarity and certainty for coastal states in understanding the extent of their national jurisdiction. For the more contentious issue of overlapping claims between neighbouring states, the law has evolved significantly. While the simple median line is the default for the territorial sea, the approach for the EEZ and continental shelf is more nuanced. Guided by the UNCLOS requirement for an "equitable solution", international courts and tribunals have established a consistent three-stage methodology. This approach balances the predictable starting point of an equidistance line with the flexibility needed to account for specific geographical and other relevant circumstances, thereby preventing an inequitable outcome. The law in this area successfully creates a durable framework that balances the rights of individual states with the need for a stable and predictable international order for the oceans.
References
Churchill, R.R. and Lowe, A.V. (2015) The Law of the Sea. 3rd edn. Manchester: Manchester University Press.
Guilfoyle, D. (2021) 'Maritime Delimitation: Law and Technique in 2021'. In: D. Guilfoyle (ed.) Research Handbook on the Law of the Sea. 2nd edn. Cheltenham: Edward Elgar Publishing.
Anglo-Norwegian Fisheries Case (United Kingdom v Norway) [1951] ICJ Rep 116.
Maritime Delimitation in the Bay of Bengal (Bangladesh/Myanmar), ITLOS Case No. 16, Judgment of 14 March 2012.
North Sea Continental Shelf Cases (Federal Republic of Germany v Denmark; Federal Republic of Germany v Netherlands) [1969] ICJ Rep 3.
Tanaka, Y. (2019) The International Law of the Sea. 3rd edn. Cambridge: Cambridge University Press.
United Nations Convention on the Law of the Sea (1982) 1833 UNTS 3.
